1. Field of the Invention
This invention relates to direct current electric furnaces of the type utilizing an electrode or electrodes extending downwardly into a vessel having a conductive hearth, in the form of an inverted arch constructed of refractories, to provide a return current path.
2. Review of the Art
Existing conducting hearth designs found in DC furnaces include:
multiple courses of conductive refractory brick over a copper plate; conducting steel pins embedded in a rammed refractory over a steel plate; conducting steel fins embedded in refractory over a steel plate; and conducting steel billets embedded in the refractory. All these designs operate with vertical current conduction through the hearth to a DC return power conductor system located beneath the furnace.
The electrically conductive brick design requires multiple courses of refractory brick having a high electrical conductivity. Electrically conductive refractory materials generally have increased thermal conductivity which results in excessive heat transfer out the furnace bottom. The thermal heat conduction together with the significant electrical power losses associated with conductive brick cause increased thermal expansion of the hearth. The expansion creates gaps in the brickwork which fill with metal fingers. The movement of the bricks also causes uneven pressures between brick courses which in turn can alter the distribution of hearth contact resistances. Since the hearth current flows through the paths of lowest resistance, local changes in contact resistance between the conductive brick and copper plate can re-direct hearth current flow. As a consequence, non-uniform current densities are created in the hearth which accelerate uneven brick movement and can eventually cause floating of loosened refractory brick. The uneven current densities also create hot spots which can re-melt frozen metal fingers in the brick. If the molten bath should become superheated, a bottom run out could easily be initiated through a current conducting metal finger in the hearth brick.
The electrical conduction paths of these existing designs are dictated by the hearth brick resistance distribution and hence contact pressure distribution between hearth bricks. Since these forces cannot be controlled in an orderly fashion, the hearth resistance distribution can vary, which in turn, can reduce the maximum current capacity of this design. Since the hearth brick and copper plate are not accessible from outside the furnace, any bottom repairs require a total furnace shutdown.
The electrically conductive steel pins, fins and billet hearth conductors are designed to operate in direct contact with the molten bath. These designs are therefore consumable and require frequent replacement. The conductive steel-refractory hearth assemblies are generally designed for scheduled replacement and require a full shutdown. Bottom conductor replacement also requires that the molten bath be completely tapped out of the furnace. Because the steel pins, fins and billets all require regular replacement, a brick inverted arch hearth, which is not designed for fast or frequent replacement, cannot be used with the steel conductors. Steel pins, fins or billets do not provide the benefits of the inverted brick hearth for smelting operations. These benefits include: a tight refractory lining which resists matte or metal penetration by hot melts or slags; the ability to apply external mechanical compressive forces which maintain the hot bath containment integrity of the hearth; and the ability to use, in non-conductive zones, electrically non-conductive refractory materials which better resist erosion or chemical dissolution by chemical reaction with the hot bath, and reduce heat transfer through the base of the hearth.
In U.S. Pat. No. 3,383,450 (Dillon et al) and U.S. Pat. No. 4,336,411 (Hanas et al) proposals have been made to provide electrodes in the side walls of an arc furnace and a melt ladle respectively, the objective in the first case being to provide auxiliary electrodes to avoid cold spots, while in the second case the application is considerably different from a conventional DC arc furnace. U.S. Pat. No. 4,204,082 (Stenkvist) also discloses a DC arc furnace with an auxiliary side-wall electrode, used only in the early stages of a melt.
U.S. Pat. No. 1,167,176 discloses the use of water cooled electrodes embedded in the hearth of an arc furnace, but very clearly such electrodes cannot be serviced without taking the furnace out of service.
U.S. Pat. Nos. 5,052,018 and 5,199,043 (Meredith) disclose DC arc furnaces with inverted arch under conductive brick hearths, to which current is supplied through an annular conductor surrounding external side walls of the hearth. Such arrangements avoid the necessity for steel or copper conductors beneath the furnace, but do not address the other problems associated with conductive brick hearths and outlined above.
U.S. Pat. No. 3,849,587 (Hatch et al) discloses the use of water cooled copper blocks embedded in the sidewall of a furnace with a view to protecting the refractory sidewall of the furnace from erosion by forming a local layer of frozen melt over the lining.